Re: Why shouldn't we have a #if .... #else .... #endif construct in Emacs Lisp?

[Ihor Radchenko]

Richard Stallman <rms@gnu.org> writes:

>   > Because there are cases when `fboundp' should be checked at runtime.
>   > For example, some parts of Org use `fboundp' to check if certain
>   > third-party optional dependencies are loaded. If `fboundp' is resolved
>   > at compile time, such checks will be broken.
>
> I am confident there is a convenient way to distinguish the cases
> where you want run-time tests from those where you would like
> compile-time tests.  But I can't try to look for it until I see
> what the former cases look like.  Can you show me an example?

Org mode uses `fboundp' check to fontify arbitrary source blocks:

#+begin_src language
...
#+end_src

The corresponding code does the following:

(defun org-src-font-lock-fontify-block (lang start end)
  "Fontify code block between START and END using LANG's syntax.
This function is called by Emacs' automatic fontification, as long
as `org-src-fontify-natively' is non-nil."
...
    (let ((lang-mode (org-src-get-lang-mode lang)))
      (when (fboundp lang-mode)
      <fontify using LANG-MODE's native fontification>

This cannot be optimized on compile time because we cannot know in
advance which languages are to be fontified by user.

Another example is Org mode optional inlinetask library "org-inlinetask".
When the library is loaded, certain constructs are parsed differently in
Org files:

(defun org-back-to-heading (&optional invisible-ok)
  "Go back to beginning of heading or inlinetask."
  (forward-line 0)
  (or (and (org-at-heading-p (not invisible-ok))
           (not (and (featurep 'org-inlinetask)
                   (fboundp 'org-inlinetask-end-p) ; <----
                   (org-inlinetask-end-p))))

This must also be a runtime check, because it is up to the users whether
this optional library is to be used or not.

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
Support Org development at <https://liberapay.com/org-mode>,
or support my work at <https://liberapay.com/yantar92>